1. Field of the Invention
The invention is directed to a mold closing device for injection molding machines, especially for injection molding machines for plastic material.
2. Description of Related Art
Injection molding machines, which are used especially to manufacture plastic parts, comprise mold closing plates that are movable relative to each other. The mold closing plates each support one half of an injection molding tool, the mold closing plates being moved towards each other to close the tool. After at least one of the two mold closing plates has been displaced by means of a displacing means, it is necessary to apply a closing force on the two mold closing plates, and thus on the two tool parts, by means of a holding means so as to prevent the two tool parts from being pressed apart when injecting the injection molding mass.
Generally, closing units with two or three plates are known. In three-plate closing units, two outer stationary mold closing plates are provided. A displaceable mold closing plate, the so-called closing plate, is provided between these two mold closing plates, which closing plate is supported with respect to one of the stationary mold closing plates and is moved towards the second stationary mold closing plate, the so-called faceplate, to close the device. The two mold closing plates that are moved towards each other when closing the device, carry the injection molding tools. With two-plate closing units, a stationary faceplate and a movable closing plate are usually provided. The device is closed by pulling the closing plate towards the stationary faceplate. On the injection side, the displacing means is arranged in a superposed or nested manner with respect to the injection unit. Therefore, two-plate closing units have a shorter structural length.
In three-plate closing units, toggle levers are known for closing the mold closing device. Using the toggle levers, the movable mold closing plate is moved first and, thereafter, a corresponding closing force is applied by the toggle lever. However, such mold closing devices are disadvantageous in that an exact displacement of the movable mold closing plate is difficult, especially during final positioning. Further, the toggle lever may be pushed too far. This results in the closing force possibly not being clearly defined. This may impair the quality of the injection molded parts. Therefore, such mold closing devices have an increased tool wear. Moreover, the bars may be overstressed.
Further, hydraulic closing units are known, wherein both the displacement of the closing plate and the application of the closing force are effected through a hydraulic displacing and holding means. To this effect, the closing means have corresponding hydraulic cylinders with pistons guided therein. Such hydraulic systems usually have at least one differential cylinder that is pressurized. Thus, it is possible to perform an approaching movement such as the closing of the mold closing device. Through a connectable further linear hydraulic cylinder, a slow opening of the closing device can be achieved by a reversal of the area ratios. Here, the closing force is achieved through a cylinder with a relatively large diameter to which high pressure can be supplied. To do so, a hydraulic pressure transmission unit can be employed that serves both as a closing valve and as a hydraulic pressure transmitting element. Such a pressure transmission unit is described, for example, in U.S. Pat. No. 5,750,163. Positioning the movable closing plate exactly is technically complex.